Various systems have been developed for controlling the draft of an implement associated with an earthworking vehicle, such as an agricultural or construction type tractor. Implement draft is generally defined as the force required to push or pull the implement, and is responsive to the depth of penetration of the implement into the soil. Most such prior systems provide controls by which an operator can select a desired implement penetration depth. This depth position is normally maintained by the control system unless some predetermined working condition requires that the depth be modified. In the most common situation, the control system monitors one or more selected working conditions, such as vehicle speed or draft force, and modifies the implement penetration in response to the working condition.
One example of such a prior control system is found in U.S. Pat. No. 4,495,577, issued Jan. 22, 1985, to Strunk, et al. Strunk teaches a system for controlling the working depth of an implement pulled by an agricultural vehicle. The system senses engine speed, implement created draft force, and implement or hitch position. Based on these sensed values, the system modifies the depth of implement penetration in an attempt to most efficiently operate the vehicle. Strunk does recognize one important factor that must be considered when attempting to automatically control implement draft. Draft control systems with fixed gain characteristics do not function well under varying soil conditions. When the soil is compact and difficult to work, a particular gain setting may be too high, resulting in implement oscillation or instability. However, the same gain setting may prove to be too low and unresponsive to different conditions in which the soil is less compact. Therefore, many draft control systems are difficult to utilize because multiple settings of gain and other control characteristics have to be made in a somewhat intuitive manner by the operator. Such systems are simply not suited to actual field working conditions.
Recognizing that stability of the control system is necessary for proper operation, Strunk considers a plurality of working condition factors in the described control algorithm. However, the Strunk system still does not achieve optimum control stability because it is only able to react after a particular error condition has been sensed, and is not able to predictively modify the characteristics of the control algorithm.
The present invention is directed to overcoming the various problems associated with implement draft control, including the shortcomings of the Strunk disclosure.